It's plain epoxy with added black colouring. Nothing magic.
But lots of epoxies have additives in them, not due to the final result, but to ease working with them.
The rectifier in your picture is super simple to seal, because you can just pour in the epoxy, and that's it.
Coating a PCB is a different story, it's not like pouring epoxy into a bowl.
Do a search for "conformal coating", that will bring you further.